Blue LEDs Open Door to Microdisplays

MANHATTAN, Kan. -- Physicists at Kansas State University have created a microdisplay based on InGaN/GaN quantum wells that is integrated on a semiconductor chip and is only 0.5 mm square. The group hopes it will pave the way for bright, high-resolution displays that can be worn like eyeglasses.

Physicists at Kansas State University have developed a blue microdisplay based on InGaN/GaN quantum wells. Courtesy of Kansas State University.
The approach has several potential advantages over conventional display technologies, said Hongxing Jiang, the lead author of a paper reporting the work in the Feb. 26 issue of Applied Physics Letters. Large flat panel displays use thousands of individual LEDs, but this is not practical for microdisplays. Liquid crystal technology typically requires an external light source. And organic LEDs, which are small and multicolored, cannot withstand the high current densities needed to create a display bright enough to compete with ambient light. The new display, however, overcomes all these drawbacks.

The key is that it is made of III-nitride, wide-bandgap semiconductors, so it operates at high powers and temperatures and is shock-resistant. It also can operate at a wide range of wavelengths, from the ultraviolet through the red.

The pixels in the 10 x 10-pixel prototype LED array are 12 µm in diameter. With the right magnification, however, they appear as large as a 21-in. television. Because they are light-emitting, they can be viewed at wide angles without losing contrast or color.

The researchers, led by Jiang and Jingyu Lin, used photolithographic patterning, inductively coupled plasma etching and ohmic contact metallization to create the LED structure in the alternating layers of semiconductor material.

The researchers grew the semiconductor on a sapphire substrate, which is transparent to visible light and therefore makes an ideal display surface. In addition, they could control pixels individually because the process creates a control pad for each.

Moving to color

The physicists believe that the technique will produce microdisplays with sufficient brightness to be integrated into wearable projection systems. "Normally, the amount of light loss in a projection system is very small," Jiang said. "There should be no problem regarding eye damage [either]."

He said that the group plans to continue developing the devices, with the goal of creating a more viable display than the single-color, blue prototype. "We will try to make them full-color, with more pixels – a few hundred by a few hundred."